Fredrick C. Hagemeister

Resonant ion-dip infrared spectroscopy of benzene–(methanol)m clusters with m=1–6

Resonant ion-dip infrared spectroscopy has been employed to record cluster-size-specific spectra of C6H6–(CH3OH)m with m=1–6 in the OH stretch fundamental region. The comparison of the spectra with the results of ab initio calculations on the pure methanol clusters enables the assignment of the hydrogen-bonding architecture in the clusters. In all cases, the methanol molecules aggregate together in a single subcluster. With m=1, a single infrared transition is observed, redshifted from that of a free methanol momomer by 42 cm−1 due to π hydrogen bonding between benzene and methanol. The m=2 spectrum features two strong transitions at 3506 and 3605 cm−1. The lower frequency peak is redshifted from the free monomer value by 175 cm−1 and is assigned to the proton donor in the methanol dimer subcluster. The proton acceptor, which would be a free OH stretch in the absence of benzene, is redshifted by 76 cm−1 due to a strengthened π hydrogen bond. In benzene–(CH3OH)3, three sharp OH stretch transitions are obse...

Resonant ion-dip infrared spectroscopy of the S4 and D2d water octamers in benzene-(water)8 and benzene2-(water)8

The techniques of resonant two-photon ionization (R2PI), UV–UV (ultraviolet) hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopies have been employed along with density functional theory (DFT) calculations to assign and characterize the hydrogen-bonding topologies of two isomers each of the benzene-(water)8 and (benzene)2(water)8 gas-phase clusters. The BW8 isomers (B=benzene, W=water) have R2PI spectra which are nearly identical to one another, but shifted by about 5 cm−1 from one another. This difference is sufficient to enable interference-free RIDIR spectra to be recorded. As with smaller BWn clusters, the BW8 clusters fragment following photoionization by loss of either one or two water molecules. The OH stretch IR spectra of the two BW8 isomers bear a close resemblance to one another, but differ most noticeably in the double-donor OH stretch transitions near 3550 cm−1. Comparison to DFT calculated minimum energy structures, vibrational frequencies, and infrared intensities leads to an a...

The hydrogen-bonding topologies of indole–(water)n clusters from resonant ion-dip infrared spectroscopy

A combination of resonant two-photon ionization, infrared-ultraviolet hole burning, and resonant ion-dip infrared spectroscopies are used to assign and selectively probe the hydrogen bonding topologies of indole–(water)n clusters with n=1,2. The indole–(water)1 complex is confirmed to possess the N−H⋯OH2 structure surmised from previous studies. However, the bands in the ultraviolet previously assigned to a π H-bound indole–water complex are shown to be due instead to the indole–(water)2 cluster in which the water dimer forms a H-bonded bridge between the N–H and aromatic π clouds of indole. The implications of this reassignment for our understanding of the influence of H-bonding solvents on indole’s fluorescence properties are discussed.

Resonant ion-dip infrared spectroscopy of benzene–(water)9: Expanding the cube

The techniques of resonant two-photon ionization (R2PI), UV-UV hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopy have been employed along with density functional theory (DFT) calculations to characterize the hydrogen-bonding topologies of three isomers of benzene–(water)9. Isomers I and II, with R2PI transitions shifted, respectively, by +77 and +63 cm−1 from the benzene monomer, have similar intensities in the R2PI spectrum. The signal from the third isomer (isomer III, shifted +60 cm−1) is present at about one-fourth the intensity of the other two. The experimental RIDIR spectrum of isomer I bears a strong resemblance to the spectrum of the benzene–(water)8 D2d-symmetry cubic structure identified in earlier work, but possessing an extra single-donor transition associated with the ninth water molecule. Using the S4 and D2d symmetry forms of the water octamer as base structures to which the ninth water molecule can be added, a total of nine “expanded-cube” structures are identified for W9 a...

FLUORESCENCE-DIP INFRARED SPECTROSCOPY OF TROPOLONE AND TROPOLONE-OD

Fluorescence‐dip infrared spectroscopy (FDIRS) is employed to record the infrared spectra of the isolated, jet‐cooled tropolone molecule (TrOH) and its singly deuterated isotopomer TrOD in the O–H and C–H stretch regions. The ability of the method to monitor a single ground‐state level enables the acquisition of spectra out of the lower and upper levels of the zero‐point tunneling doublet free from interference from one another. The high power of the optical parametric oscillator used for infrared generation produces FDIR spectra with good signal‐to‐noise despite the weak intensity of the C–H and O–H stretch transitions in tropolone. The expectation that both spectra will exhibit two OH stretch transitions separated by the OH(v=1) tunneling splitting is only partially verified in the present study. The spectra of TrOH are compared with those from deuterated tropolone (TrOD) to assign transitions due to C–H and O–H, which are in close proximity in TrOH. The appearance of the spectra out of lower (a1 symmet...

Vibrational spectroscopy of methanol and acetonitrile clusters in cold helium droplets

Infrared molecular beam depletion spectroscopy of small methanol and acetonitrile clusters embedded in large helium clusters has been studied in the spectral region of the CO stretch and the CH3 rock mode from 1023 to 1059 cm−1. The results are compared with the experimental spectra of the corresponding free clusters generated in adiabatic expansions and calculations based on density functional theory or empirical potential models. For methanol clusters, the two types of experimental results are the same for the dimer and trimer structure. Different isomers are found in cold helium for the tetramer and pentamer, namely a monomer and dimer attached to a cyclic trimer. For acetonitrile clusters in helium, aside from the dimer, different structures are observed. The spectra from the trimer to the hexamer are dominated by structures which contain the antiparallel dimer as building block with D2d symmetry for the tetramer. They do not correspond to the minimum configurations observed for the free clusters. The...

Fluorescence‐dip infrared spectroscopy of the tropolone‐H2O complex

Fluorescence dip infrared spectroscopy (FDIRS) is used to probe the effect of a solvent water molecule on intramolecular H‐atom tunneling in tropolone. As with the bare molecule discussed in paper I, the FDIR spectrum of the tropolone‐H2O complex is recorded in the O–H and C–H stretch regions. Three OH stretch fundamentals are observed in the spectrum, and can be assigned nominally to a free OH stretch of the water molecule (3724 cm−1), a hydrogen bonded OH stretch of water (3506 cm−1), and the OH stretch of tropolone (∼3150 cm−1). The breadth and complexity of the bands is highly mode specific. The free OH stretch transition is sharp (1.8 cm−1 FWHM) and has weak combination bands built on it at +73 and +1600 cm−1. The former is assigned to a combination band with the in‐plane bending mode of the tropolone‐H2O hydrogen bond, while the latter is the free OH/intramolecular water bend combination band. The water hydrogen‐bonded OH fundamental is also a sharp transition which, after correction for the decreas...

RESONANT ION-DIP INFRARED SPECTROSCOPY OF TERNARY BENZENE-(WATER)N(METHANOL)M HYDROGEN-BONDED CLUSTERS

Resonant two-photon ionization, IR−UV hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopies have been employed along with density functional theory (DFT) calculations to assign and characterize the hydrogen-bonded topologies and structures of eight benzene−(H2O)n(CH3OH)m, cluster isomers (hereafter shortened to BWnMm) with n + m ≤ 4. The O−H stretch infrared fundamentals are used to determine the H-bonding topology of the clusters. However, in several cases, the O−H stretch spectrum leaves an ambiguity regarding the position of the methanols within the structure. In these cases, the methyl CH stretch region serves as a secondary probe capable of distinguishing among the various possibilities. For n + m = 2, a single BWM isomer is observed with OH stretch fundamentals at 3508, 3606, and 3718 cm-1. A comparison of the methyl CH stretch transitions of BWM and BM2 reveals that the methanol in BWM accepts a H-bond from water and forms a π H-bond with benzene. The n + m = 3 results show the subtle ...

Resonant ion-dip infrared spectroscopy of benzene–(water)n–(methanol)m clusters with n+m=4, 5

Abstract Resonant two-photon ionization and resonant ion-dip infrared (RIDIR) spectra of benzene–(water) n –(methanol) m clusters (hereafter shortened to BW n M m ) have been recorded for a total of seven clusters with n + m =4 and 5. The infrared spectra in the OH and CH stretch regions show absorptions characteristic of H-bonded W n M m clusters which are bound to benzene by a π H-bond involving a dangling OH on the W n M m sub-unit. Density functional theory (DFT) calculations identify a number of conformational isomers in the n + m =4 series which meet the general criteria imposed by the experimental spectra. The structures, binding energies, harmonic vibrational frequencies, and infrared intensities for these isomers have been calculated for comparison with experiment. Based on the calculations, tentative assignments of several of the observed species are given. The calculations uncover the fact that complexation of benzene to the cyclic water tetramer imposes much the same perturbations on the cycle as substitution of methanol for water. In particular, the single-donor OH stretch spectra of W n M m and BW n +1 M m −1 are calculated to be virtually identical to one another. The comparison of experiment and theory for this series of cyclic structures is used to assess the strengths and limitations of the calculations at the DFT Becke3LYP/6-31+G * level of theory.